Microbiology #3

Relationship Between Maternal and Neonatal Staphylococcus aureus Colonization

WHAT’S KNOWN ON THIS SUBJECT: Staphylococcus aureus is a leading cause of infections in infants. Staphylococcal colonization is a known risk factor for infection, but whether maternal colonization plays a role in subsequent colonization in the infant is unclear.

WHAT THIS STUDY ADDS: This prospective study found that infants born to women colonized with S aureus either during their third trimester of pregnancy or at the time of delivery are more likely to harbor S aureus than are those born to noncolonized women.

abstract OBJECTIVE: The study aimed to assess whether maternal colonization with Staphylococcus aureus during pregnancy or at delivery was associated with infant staphylococcal colonization.

METHODS: For this prospective cohort study, women were enrolled at 34 to 37 weeks of gestation between 2007 and 2009. Nasal and vaginal swabs for culture were obtained at enrollment; nasal swabs were obtained from women and their infants at delivery and 2- and 4-month postbirth visits. Logistic regression was used to determine whether maternal colonization affected infant colonization.

RESULTS: Overall, 476 and 471 mother-infant dyads had complete data for analysis at enrollment and delivery, respectively. Maternal methicillin-resistant S aureus (MRSA) colonization occurred in 10% to 17% of mothers, with the highest prevalence at enrollment. Infant MRSA colonization peaked at 2 months of age, with 20.9% of infants colonized. Maternal staphylococcal colonization at enrollment increased the odds of infant staphylococcal colonization at birth (odds ratio; 95% confidence interval: 4.8; 2.4–9.5), hospital discharge (2.6; 1.3–5.0), at 2 months of life (2.7; 1.6–4.3), and at 4 months of life (2.0; 1.1–3.5). Similar results were observed for maternal staphylococcal colonization at delivery. Fifty maternal-infant dyads had concurrent MRSA colonization: 76% shared isolates of the same pulsed-field type, and 30% shared USA300 isolates. Only 2 infants developed staphylococcal disease.

CONCLUSIONS: S aureus colonization (including MRSA) was extremely common in this cohort of maternal-infant pairs. Infants born to mothers with staphylococcal colonization were more likely to be colonized, and early postnatal acquisition appeared to be the primary mechanism. Pediatrics 2012;129:e1252–e1259

AUTHORS: Natalia Jimenez-Truque, MQC, MSCI,a Sara Tedeschi, MD,b Elizabeth J. Saye, BS,a Brian D. McKenna, BS,a Weston Langdon, BS,a Jesse P. Wright, BS,a Andrew Alsentzer, BS,a Sandra Arnold, MD, MS,c Benjamin R. Saville, PhD,d Wenli Wang, MS,d Isaac Thomsen, MD,a and C. Buddy Creech, MD, MPHa

aDivision of Pediatric Infectious Diseases, Vanderbilt University Medical Center, Nashville, Tennessee; bDepartment of Medicine, Brigham and Women’s Hospital, Boston, Massachusetts; cDivision of Infectious Diseases, University of Tennessee Health Science Center, Memphis, Tennessee; and dDepartment of Biostatistics, Vanderbilt University School of Medicine, Nashville, Tennessee

KEY WORDS Staphylococcus aureus, child, colonization, epidemiology, pregnancy

ABBREVIATIONS CA-MRSA—community-associated methicillin-resistant Staphylo- coccus aureus GBS—group B Streptococcus GEE—generalized estimating equation MRSA—methicillin-resistant Staphylococcus aureus MSSA—methicillin-susceptible Staphylococcus aureus PCR—polymerase chain reaction PVL—Panton-Valentine leukocidin SSTIs—skin and soft-tissue infections VUMC—Vanderbilt University Medical Center

All authors have contributed appropriately to meet the criteria to warrant authorship.

www.pediatrics.org/cgi/doi/10.1542/peds.2011-2308

doi:10.1542/peds.2011-2308

Accepted for publication Dec 15, 2011

Address correspondence to Natalia Jimenez-Truque, MQC, MSCI, Epidemiology Graduate Student, Pediatric Infectious Diseases, Vanderbilt University Medical Center, 1161 21st Ave South, D-7215 MCN, Nashville, TN 37232. E-mail: [email protected]

PEDIATRICS (ISSN Numbers: Print, 0031-4005; Online, 1098-4275).

Copyright © 2012 by the American Academy of Pediatrics

FINANCIAL DISCLOSURE: The authors have indicated they have no financial relationships relevant to this article to disclose.

FUNDING: This work was supported by the Fogarty International Center at the National Institutes of Health (grant 1 R25 TW007697), the 2007 IDSA/SHEA Young Investigator Award in MRSA Research (Dr Creech), the Monroe Carell, Jr. Children’s Hospital Fund, and the National Center for Research Resources at the National Institutes of Health (grant 1 UL1 RR024975). Funded by the National Institutes of Health (NIH).

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Methicillin-resistant Staphylococcus au- reus (MRSA) causes ∼100 000 inva- sive infections and ∼20 000 deaths per year in the United States; of these, ∼1000 infections and ∼100 deaths oc- cur in children ,1 year of age.1 Further, MRSA causes between 59% and 72% of all skin and soft-tissue infections (SSTIs),2,3 and up to 95% of all SSTIs in children are caused by community- associated MRSA (CA-MRSA).4 MRSA now affects previously healthy individ- uals without known risk factors,5,6 and CA-MRSA has become the most frequent clone of S aureus in many communities, causing disease in neonatal intensive care units (NICUs)7,8 and even among healthy full-term babies.9,10

Approximately one-third of the gen- eral population carries S aureus in their nares,11 and staphylococcal col- onization is a known risk factor for subsequent infection.11 Previous data suggest that the frequency of MRSA colonization ranges from 1% to 4% in infants and mothers,7–9,12–16 though some areas of the US experience higher rates of colonization.17–19 While known risk factors for infant S aureus colonization include breastfeeding, number of household members,20,21

low birth weight, early gestational age at birth, indwelling catheters, and du- ration of antibiotic or ventilator days,7

it is not clear whether maternal nasal and anogenital colonization plays a role in infant colonization. Colonized moth- ers can transmit MRSA to their infants,10,12,13 but it remains unclear whether there is real potential for sig- nificant vertical maternal-infant trans- mission of MRSA.

Our objective was to determine the clinical and molecular epidemiology of staphylococcal colonization in mothers and their infants from the third tri- mester of pregnancy to 4 months after birth. By obtaining nasal swabs at each time point, we estimated the frequency of staphylococcal colonization and

analyzed the molecular characteristics of these isolates. We also sought to examine whether maternal MRSA nasal and/or vaginal colonization is associ- ated with subsequent colonization or infection in the infant.

METHODS

Study Population

We conducted a prospective study of MRSA colonization in a cohort of maternal-infant pairs between June 2007andMarch2009.Weinvitedwomen who were in their third trimester of pregnancy (34–36 weeks of gestation) and cared for at the Obstetrics Clinic of Vanderbilt University Medical Center (VUMC) in Nashville, Tennessee, or the UT Medical Group Obstetrics Clinic at the University of Tennessee Health Science Center in Memphis, Tennessee, to participate. Women had to be .18 years of age, willing to comply with study-related procedures (including na- sal swabs, enrollment of her child when born, and willingnessto attend follow-up visits), and capable of providing written informed consent. The local institutional review boards of VUMC and University of Tennessee Health Science Center ap- proved the study.

Study Procedures

An in-person interview questionnaire was administered to determine risk factors for staphylococcal exposure/ carriage, and a moistened nasal swab was collected from the mother at the time of enrollment and on the day of delivery. Additionally, the group B Streptococcus (GBS) culture collected from the mother during her routine prenatal care was sampled to detect S aureus.

After the infant was born, the nursing staff of the newborn nursery or NICU alertedstudypersonnelwithin2hoursof delivery. Cultures of nares and umbilicus were obtained with a moistened cotton

swab before triple dye was applied to the umbilicus. For newborns, cultures were repeated immediately before discharge.

After discharge, infants and mothers enrolled at VUMC were asked to return to the Pediatric Clinical Research Cen- terfornasalswabculturesamplestobe taken at 2 and 4 months of age, while samples from Memphis were collected only if participants voluntarily returned. Questionnaires were administered at each visit to assess risk factors for staphylococcal exposure and carriage, history of maternal/infant staphylococ- cal infection, history of hospitalization or other medical illnesses/procedures, and antibiotic use. Medical charts were reviewed for clinically relevant illnesses consistent with staphylococcal infec- tions.Motherswereinstructedtoalert study personnel if their infants devel- opedSSTIsoriftheyor theirinfantswere hospitalizedforany reason. This allowed for additional cultures to be obtained, where appropriate.

Cultures and Molecular Laboratory Testing

All samples collected in this study were processedatVUMC.Nasalandumbilical swabs were placed in tryptic soy broth with 6.5% NaCl and incubated for 24 hours at 37°C as an enrichment step. Vaginal swabs were first processed at the VUMC Microbiology Laboratory and inoculated into Lim Broth (Becton, Dickinson, and Co, Franklin Lakes, NJ) for the detection of GBS. After broth enrichment of all samples, a 10-mL in- oculum was plated onto mannitol salt agar plates with and without 4 mg/mL oxacillin and incubated for 48 hours at 37°C. If yellow growth was observed, colonies were plated onto tryptic soy agar with 5% sheep blood and incu- bated for 24 hours at 37°C. Latex ag- glutination testing was performed for the detection of clumping factor (Staphaurex; Remel, Lenexa, KS), and the presence of the nuc gene (specific

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to S aureus) was confirmed by poly- merase chain reaction (PCR). Isolates confirmed to be MRSA by PCR detection of the mecA gene were further char- acterized by SCCmec typing, by using the multiplex strategy of Oliveira and de Lencastre.22 Nontypeable isolates by the multiplex strategy underwent ccr and mec class typing as previously described.23 Detection of the Panton- Valentine leukocidin (PVL) gene locus was performed, as described else- where.24 Genotyping of MRSA isolates was performed by repetitive element sequence–based PCR.25 Isolates with .95% similarity were defined as in- distinguishable.

Statistical Methods

Wilcoxon rank sum tests and Pearson x2 tests were used to compare patient characteristics between the 2 study centers and between those with and without infant colonization at birth. Correlations between patient charac- teristics were assessed by using Spearman’s correlation coefficient, and pairs of variables with a high correla- tion were noted before proceeding to modeling. Logistic regression models with generalized estimating equations (GEEs) were used to model child colo- nization as a function of maternal colo- nization at birth or enrollment and time since birth (birth, discharge, 2 months, 4 months), adjusting for the following potential confounders: number of pre- vious births, gestational age at enroll- ment, mode of delivery, race, and admission to the NICU. The model also included an interaction term between maternal colonization at birth or en- rollment and time. Due to potential col- linearity, separate logistic regression models were fitted with either maternal colonization at birth or maternal colo- nization at enrollment as predictors. A sensitivity analysis was conducted in which both GEE models were fitted by using data from patients enrolled at VUMC alone.

RESULTS

Overall, 629 mother-infant dyads were enrolled. Demographic data and clinical history are displayed in Table 1. Infants colonized at birth were more likely to have an African American mother (75% vs 41%, P , .001) and to have been born vaginally (86% vs 69%, P = .018). No other patient character- istics were associated with infant staph- ylococcal colonization at birth (data not shown). No preterm deliveries were en- countered as we preferentially enrolled women close to term and having GBS screening performed. Thus, all infants were born at or greater than 37 weeks’ gestation. The average time between enrollment and delivery was 5 weeks,

corresponding to enrollment at 34 to 36 weeks and delivery at term.

Comparison Between the Two Study Centers

To determine whether demographic characteristics were similar between the 2 study centers, we compared pa- tient characteristics between the 2 sites. Women enrolled in the VUMC co- hortwereolder(median,26vs23years; P , .001), had higher median ges- tational ages at enrollment (median, 36 vs 35 weeks; P , .001), were less likely to undergo caesarean section (27% vs 37%, P = .008), and were more likely to have a history of mastitis or staphylococcal infections previous to

TABLE 1 Demographic and Clinical Characteristics, by Enrollment Center

Na Vanderbilt (N = 399)

Memphis (N = 230)

Total (N = 629)

P

Gestation, median wk (interquartile range)

629 36 (36–36) 35 (35–36) 35 (35–36) ,.001b

Maternal race, n (%) 629 ,.001c

African American 132 (33) 203 (88) 335 (53) Asian 4 (1) 0 (0) 4 (1) Caucasian 228 (57) 22 (10) 250 (40) Hispanic 12 (3) 5 (2) 17 (3) Other 23 (6) 0 (0) 23 (4)

Maternal age, median y (interquartile range)

620 26 (21–30) 23 (21–27) 24 (21–29) ,.001b

Previous births, median n (interquartile range)

629 1 (0–2) 1 (0–2) 1 (0–2) .014b

Mode of delivery, n (%) 594 .008c

Cesarean section 101 (27) 79 (37) 180 (30) Vaginal 279 (73) 135 (63) 414 (70)

Infant admitted to, n (%) 594 .15c

NICU 16 (4) 15 (7) 31 (5) Nursery 362 (96) 201 (93) 563 (95)

History of mastitis, n (%) 627 .011c

No 383 (96) 228 (100) 611 (97) Yes 15 (4) 1 (0) 16 (3)

Previous maternal hospitalizations/ surgeries, n (%)d

629 ,.001c

No 169 (42) 67 (29) 236 (38) Yes 230 (58) 163 (71) 393 (62)

Previous maternal staphylococcal infections, n (%)d

629 .016c

No 376 (94) 226 (98) 602 (96) Yes 23 (6) 4 (2) 27 (4)

a N is the number of observations (missing values account for numerical differences between groups). b Wilcoxon rank-sum test. c Pearson x2. d Within the past 12 mo before enrollment.

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enrollment (3.8% vs 0.4%, P = .011; 6% vs 2%, P = .016), respectively (Table 1).

Colonization Prevalence Over Time

The proportions of staphylococcal col- onizationinmothersandtheirinfantsat each time point are shown in Fig 1. Maternal MRSA colonization was high- est at enrollment, with 225 (38.6%) women colonized with S aureus and 97 (16.6%) colonized with MRSA (75 in the nares alone, 15 with anogenital coloni- zation alone, and 7 with both). At de- livery, 136 and 53 women were colonized with S aureus and MRSA, respectively. At 2 and 4 months, 110 and 72 mothers had S aureus, and 41 and 39 had MRSA, respectively.

At birth, 44 (9.3%) infants were colo- nized with S aureus and 12 (2.5%) with MRSA; nasal and umbilical swabs were cultured together. These frequencies were 41 and 24 at discharge, respec- tively. Infant staphylococcal coloniza- tion peaked at 2 months of age, with 121 (38.9%) infants colonized with S aureus and 65 (20.9%) with MRSA, but decreased again by 4 months, to 65 and 31, respectively.

Association Between GBS and S aureus Colonization

Of 616 women with GBS culture results, 171 (27.8%) were colonized with GBS. Of these, 69 (40.3%) demonstrated vaginal S aureus colonization (positive corre- lation, Spearman’s correlation P = .026). The majority of women without GBS colonization also lacked S aureus vagi- nal colonization (326 of 390 [83.6%]). Thus, there was no inhibitory relation- ship observed between GBS and vaginal S aureus colonization at enrollment. Among those colonized with GBS, the proportions of women with nasal and vaginal S aureus were not significantly different (33.3% vs 25.6%, respectively; P = .15), and there was a positive cor- relation between vaginal and nasal col- onization with S aureus at enrollment (Spearman’s correlation P = .024).

Incidence of Infection

Only 2 staphylococcal infections were observed during the study (0.42% of infants). One infant developed a skin abscess, while another infant developed purulent conjunctivitis. Both infections occurred near the 2-month visit, and both were caused by USA300, SCCmec IV CA-MRSA isolates; only 1 of these iso- lates contained PVL.

Association Between Maternal Colonization and Infant Colonization

To determine whether maternal coloni- zation correlated with future infant col- onization, we fit 2 GEElogistic regression models. The first evaluated maternal colonizationatenrollment(34–37weeks’ gestation) as a predictor, while the second evaluated maternal coloniza- tion at delivery. The models included 476 and 471 maternal-infant dyads with complete data, respectively (Table 2), and both analyses demonstrated that maternal staphylococcal colonization significantly increased the odds of infant staphylococcal colonization at all time points, as shown in Table 2.

There was lack of evidence for associ- ations between maternal character- istics and infant colonization at any of the time points. When performing the same analyses with only the data from the center with more complete follow- up, results did not change materially (data not shown). It should also be noted that the odds ratios for infant colonization are slightly different for maternal colonization at enrollment

FIGURE 1 Proportionofcolonizationover time.PeakageofcolonizationwithSaureus(MSSAandMRSA),aswellas MRSAspecifically,wasat2monthsofage,thoughsomeinfantswerecolonizedwithin2hoursofdelivery. Maternal colonization was stable over time, although maternal swabs were not obtained on discharge from the hospital (note nonconnecting lines).

TABLE 2 Odds of Infant Colonization With S aureus (Both MSSA and MRSA at Various Time Points), Based on Maternal Colonization at Enrollment or Delivery, Based on GEE Logistic Regression Models

OR 95% CI P Value

Maternal colonization at enrollment (n = 476) Birth 4.79 2.41–9.53 ,.001 Discharge 2.59 1.34–5.00 .005 2 mo 2.65 1.64–4.29 ,.001 4 mo 1.95 1.09–3.50 .025

Maternal colonization at delivery (n = 471) Birth 3.77 2.00–7.11 ,.001 Discharge 2.76 1.43–5.33 .002 2 mo 3.09 1.80–5.32 ,.001 4 mo 3.53 1.87–6.68 ,.001

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versus maternal colonization at de- livery. This may be due to transient maternal carriage, which could differ in the few weeks between enrollment and delivery, or to different numbers of available pairs for each analysis, which would affect the precision of the odds ratio estimate.

Giventhecorrelationbetweenmaternal colonization and infant colonization, we evaluated the characteristics of all maternal-infant dyads with concurrent colonization (Table 3). Twenty maternal- infant pairs were identified in which there was maternal colonization dur- ing pregnancy (enrollment) and infant colonization within 2 hours of birth. Eight of these pairs demonstrated maternal nasal S aureus colonization at enrollment (of which, 3 were MRSA), while 12 mothers exhibited vaginal S aureus colonization at enrollment (of which, 3 were MRSA); only 2 of the latter had indistinguishable isolates, suggesting vertical transmission. Next, we identified additional maternal-infant

pairs in which there was maternal col- onization at the time of delivery and in- fant colonization within 2 hours of birth or at discharge. Of women colonized with S aureus at delivery, 20 (4.2%) had infantswho were alsocolonizedatbirth, and 14 (3.0%) had infants colonized at discharge. MRSAwas present in 7 (35%) of 20 and 8 (57%) of 14, respectively. Concurrent colonization peaked at 2 months of age, with 51 maternal-infant dyads (16.4%, MRSA present in 19 [37%] of 51); however, concurrent colonization decreased by 4 months of age, with 23 dyads exhibiting colonization (9.3%, MRSA present in 10 [43%] of 23).

Molecular Characteristics of MRSA Isolates

Overall, 369 MRSA isolates were re- coveredfromthe cohort.SCCmec IVwas present in 74.8% of isolates, consistent with the largely community-based na- ture of the cohort. SCCmec types II, III, I, and V represented 8.1%, 7.3%, 5.4%, and 2.4% of the MRSA isolates, respectively.

Based on repetitive-element sequence- based PCR, all isolates matched 1 of 11 distinct USA pulse types recognized. The most common type was USA300, repre- senting 34.2% of all MRSA isolates; USA700 (14.4%) was also frequently identified. Of all isolates, 111 (30%) contained genes encoding PVL, an exo- toxin found predominantly in CA-MRSA strains within the USA300 pulse type; 82% of these were USA300. However, 27% of the USA300 isolates did not carry PVL genes. Variants of PFGE pulse types were not formally assessed, though variants within each pulse-type were appreciated based on rep-PCR patterns.

Of the 50 maternal-infant dyads with concurrentMRSAcolonization,38(76%) shared isolates of the same USA type (Table 3); 28 pairs (56%) carried indis- tinguishable isolates, while 15 (30%) shared USA300 isolates. Of the 78 dyads with concurrent MSSA colonization, 43 (55.1%) carried isolates of the same USA type (Table 3); 27 (34.6%) of these pairs had indistinguishable isolates,

TABLE 3 Concurrent Colonization With S aureus (MSSA and MRSA) in Maternal-Infant Pairs

Total Concurrent S. aureus (n = 128), No. (%)

Concurrent MSSA (n = 78), No. (%)

Concurrent MRSA (n = 50), No. (%)

Concurrent Colonization

Same USA Typea

.95% Similaritya

Concurrent Colonization

Same USA Typea

.95% Similaritya

Concurrent Colonization

Same USA Typea

.95% Similaritya

Enrollment nasal (n = 473)b

8 (1.7) 4 (50.0) 2 (25.0) 5 (1.1) 3 (60.0)e 1 (20.0) 3 (0.6) 1 (33.3)k 1 (33.3)

Enrollment vaginal (n = 473)c

12 (2.5) 5 (41.7) 2 (16.7) 9 (1.9) 3 (33.3)f 1 (11.1) 3 (0.6) 2 (66.7)l 1 (33.3)

Delivery (n = 473) 20 (4.2) 10 (50.0) 8 (40.0) 13 (2.7) 5 (38.5)g 4 (30.8) 7 (1.5) 4 (57.1)m 4 (57.1) Discharge (n = 462)d 14 (3.0) 11 (78.6) 7 (50.0) 6 (1.3) 4 (66.7)h 3 (50.0) 8 (1.7) 7 (87.5)n 4 (50.0) 2 mo (n = 311) 51 (16.4) 37 (72.5) 22 (43.1) 32 (10.3) 21 (65.6)i 10 (31.2) 19 (6.1) 16 (84.2)o 12 (63.2) 4 mo (n = 246) 23 (9.3) 15 (65.2) 14 (60.9) 13 (5.3) 7 (53.8)j 8 (61.5) 10 (4.1) 8 (80.0)p 6 (60.0) Total (N = 2438) 128 (5.2) 82 (64.1) 55 (43.0) 78 (3.2) 43 (55.1) 27 (34.6) 50 (2.0) 38 (76.0) 28 (56.0) a Percentages in these columns refer to the numbers in the respective “Concurrent Colonization” columns. b Pairs of maternal nasal colonization at enrollment and infant colonization at birth. c Pairs of maternal vaginal colonization at enrollment and infant colonization at birth. d Pairs of maternal colonization at delivery and infant colonization at discharge. e USA400 (3). f USA300 (2), USA700 (1). g USA200 (1), USA400 (2), USA600 (1), USA700 (1). h USA200 (2), USA400 (1), USA600 (1). i USA200 (6), USA300 (3), USA400 (6), USA500 (1), USA600 (2), USA800 (1), USA900 (1), USA1000 (1). j USA200 (3), USA400 (1), USA600 (1), USA700 (2). k USA300 (1). l USA100 (1), USA300 (1). m USA100 (1), USA300 (3). n USA200 (2), USA300 (2), USA400 (1), USA500 (1), USA800 (1). o USA200 (1), USA300 (4), USA500 (5), USA600 (4), USA700 (1), USA800 (1). p USA300 (4), USA400 (2), USA700 (2).

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and 5 (6.4%) were USA300. The propor- tion of concurrently colonized maternal- infant pairs did not differ significantly between the 2 centers (P = .1447; data not shown).

DISCUSSION

In this prospective study of women and their newborn infants, we found that infants born to women colonized with S aureus during pregnancy or at the time of delivery were more likely to be colonized with Saureus in the immediate newborn period. Though vertical trans- mission occurred, based on concurrent maternal vaginal colonization and early infant colonization, horizontal trans- mission in early neonatal life appeared to be more common. MRSA colonization was detected frequently in mothers, ranging from 10% to 16% during preg- nancy and the early postpartum period. For infants, MRSA carriage peaked at 2 months of age (20%), and more than one-third of all MRSA isolates belonged to the current epidemic clone USA300.

The prevalence of colonization with S aureus, and MRSA in particular, was higher in our cohort than has been shown in previous studies.12,14,16,26,27

Others have reported that ∼2% to 5% of mothers12,14,16,26,27 and∼1%ofinfants26,27 are colonized with MRSA. In our cohort, nearly 10% of mothers and 2.5% of infants were colonized with MRSA at the time of delivery (Fig 1). Previous studies from our group have found similar col- onization rates in mothers and chil- dren,17 suggesting that colonization in our target population may be higher than in other populations or that the method of detection (including broth enrichment prior to primary plating28) may increase yield.

This study represents the largest prospective study of maternal-infant staphylococcal colonization since the emergenceofUSA300CA-MRSA.Peacock et al, studying 100 mother-infant pairs during the 6 months after delivery,

demonstrated that one of the major determinants of infant staphylococcal carriage is maternal colonization20; at any time point, the odds of infant col- onization were nearly 4 times greater when mothers were also colonized than when mothers were not colonized. Studies from Lebon et al demonstrate similar results,29 and extend these findings to older children as well, by using the powerful Generation R Study cohort in the Netherlands to evaluate the relationship between maternal and child colonization with S aureus.30 At 24 months of age, children were twice as likely to be colonized with S aureus when the mother was also colonized. Additionally, we found that 43% of con- currently colonized dyads had indis- tinguishable isolates. A previous study by Huang et al (2009) found that at least half of all S aureus isolates in their co- hort were genetically indistinguishable, though the number of concordant maternal-infant pairs was very small.26

Taken together, these data demon- strate the important role of horizontal transmission in pediatric staphylococ- cal colonization.

This study also begins to answer a fundamental question of maternal- infant health, Does vaginal coloni- zation with S aureus during the third trimester of pregnancy portend the same risk of neonatal disease as other pathogens (eg, group B Streptococ- cus)? Our study suggests that vertical transmission occurs, as infants born to vaginally colonized mothers were 5 times more likely to be colonized within 2 hours of birth (data not shown); only in a minority of cases, however, were these strains indistinguishable. This vertical transmission, occurring in only 2 neonates, is overshadowed by the higher frequency of early horizontal transmission in infants born to mothers without vaginal colonization. The lack of inhibition of vaginal S aureus growth by GBS, as seen in other studies,13,15,16,18

did not explain the observed low rate of vertical transmission of S aureus. This implies that while hospital-based screening and interventions may be useful during staphylococcal outbreaks,8

it is horizontal spread within family members, especially mother to child, that may be a more appropriate target of intervention.

Based on SCCmec typing, CA-MRSA isolates were most prevalent (74.8%). Given the prevalence of CA-MRSA in the United States, this is not surprising; however, only 30% of these strains carried PVL, a bicomponent exotoxin found predominantly in USA300 CA- MRSA. This unexpected finding illus- trates the heterogeneous nature of staphylococci, since PVL was at one point considered pathognomonic of CA-MRSA. The identification of SCCmec IV, PVL-negative MRSA is not unique to this study; rather, previous studies from our group and others17,26,31–33

have found similar strain types in the community. This raises important ques- tions regarding the link between colo- nization and infection, generating the hypothesis that only particular strain types, containing a specific combination of virulence determinants, are best suited to cause disease in otherwise healthy individuals. Whether this is due to the independent effects of these virulence determinants (eg, effect of specific exotoxins on host cells), the overexpression of these virulence fac- tors, or a lack of host immunity to spe- cific strain types is completely unclear. The latter hypothesis is particularly in- triguing for this cohort given the likely presence of maternally derived staphy- lococcal antibodies, which could poten- tially abrogate the risk for infection in this otherwise vulnerable population. Future studies should focus on the mo- lecular characteristics of colonization strains and the prevalence of staphylo- coccal antibodies in newborns to fur- ther define this relationship.

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There are a few limitations that should be discussed. Postdischarge follow-up occurred at only 1 center, and loss to follow-up occurred. However, this loss to follow-up was nondifferential, mean- ingitoccurredsimilarlyincolonizedand noncolonized dyads, and thus is unlikely tohavesubstantiallyaffectedtheresults. Last, only nasal swabs were obtained from mothers and their infants, except at enrollment and delivery, respectively. Ourgroupandothershavedemonstrated thatstaphylococcalcarriage,particularly USA300,34,35 may occur preferentially at extranasal sites. Therefore, our data may actually be an underrepresentation of MRSA carriage in infants.

CONCLUSIONS

In this large, prospective cohort study, we identified horizontal maternal-infant transmissionastheprimarymechanism for early infant staphylococcal coloniza- tion. Vertical transmission occurs, but the efficiency of transmission appeared to be low given the relatively high fre- quencyofcarriageinmotherscompared withnewborns.Whileinfantcolonization with MRSA was common, the frequency of USA300, SCCmec IV, PVL-positive MRSA colonization was less frequent. This has important implications in disease pathogenesis given that the rates of infant staphylococcal disease in our

cohort (despite high carriage rates) were extremely low. Taken together, it would appear that prevention meas- ures focused on controlling the spread of specific strain types of MRSA (rather than all MRSA) could be a more effective strategy when outbreaks of staphylo- coccal disease in newborns occur. Fu- ture work should seek to elucidate the potential role of maternally derived antibodies in modifying staphylococcal carriage/infection risk in infants.

ACKNOWLEDGMENTS The authors are particularly grateful to the nurses and clinical research staff of theVanderbiltVaccineResearchProgram.

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